This invention relates to power amplifiers. More particularly, the invention relates to power amplifiers which efficiently amplify one or more input signals with a large dynamic range while producing low electromagnetic emissions.
A music audio signal or a movie soundtrack typically has a large dynamic range. Such signals often have a peak-to-average magnitude ratio of 8-to-1 or even higher. In addition, peaks in such signals are relatively infrequent and at most times, the signal has a magnitude close to its average magnitude. A power amplifier for such a signal must be capable of producing an output signal with corresponding high peaks and with a comparatively low average magnitude. A number of power amplifiers are known which vary the power supplied to the power amplifier""s main amplification circuit (or amplifier) so that there is limited headroom between the supplied power and the magnitude of the power amplifier""s output signal.
For example, U.S. Pat. No. 3,772,606 describes a linear class H power amplifier with four fixed power rails. Two of the rails are used to supply power to the amplifier of the power amplifier during positive half waves of the output signal and the other two rails are used to supply power to the power amplifier during negative half waves of the output signal. Of the two rails used to supply the main amplifier during the positive half waves of the output signal, one is a low voltage rail and the other is a high voltage rail. Only the low voltage rail is used to power the amplifier when the output signal has a magnitude well below that of the low voltage rail. As the output signal approaches the low voltage rail, the high voltage rail is turned on to supply additional power. This device effectively reduces the average voltage drop (i.e. the headroom) from the supply rail to the output signal, thereby improving the efficiency of the power amplifier. However, this solution is far from ideal, especially where the output signal has an average level substantially less than the lower supply rail, or slightly higher than the lower supply rail. In either case, there will still be substantial headroom between the output signal and the power supplied to the amplifier.
U.S. Pat. No. 4,430,625 describes a power amplifier that addresses this problem by providing low voltage rails which have a variable magnitude proportional to the magnitude of the output signal. The low voltage rails are provided by a switching regulator and their magnitude is controlled using a fixed frequency pulse width modulated (PWM) control signal. When the low voltage rails are insufficient to power the amplifier, high voltage rails provided by fast acting linear regulators are utilized to make up for the deficiency. This device further reduces the headroom between the output signal and the power supply to the amplifier when the output signal is lower than the low voltage rails. However, it is susceptible to high electromagnetic interference (EMI) emissions due to its hard-switching low voltage regulators. In addition, this device has no mechanism for predicting the power required by the amplifier to generate the output signal at any particular time. This results in the power supply from the switching regulators being deficient when the input signal rises rapidly and in the worst case may cause the switching regulators to be deficient during every half wave (or during many half waves) of the output signal. This in turn leads to overuse of the linear regulators, increasing the power consumption of the power amplifier and decreasing its overall efficiency.
U.S. Pat. No. 5,347,230 describes a power amplifier which attempts to reduce the usage of the linear regulators by monitoring the current in the linear regulator and controlling the output of the switching regulator in a way that minimizes the current drawn from the linear regulator. The control circuit of this power amplifier is responsive to changes in the output signal to vary the power provided by the switching regulators. This design, which is responsive rather than predictive, leads to a slow response time for the switching regulators, possibly resulting in increased usage of the linear regulators. Furthermore, this device utilizes fast-switching switching regulators which generate large EMI emissions. In addition, this device suffers from a load dumping problem which may force a high current from a current source through a high impedance load, resulting in a large voltage spike across the load.
None of these devices is well suited for use with multiple channels. Most modern audio amplifiers produce at least five output channels (i.e. surround sound systems) and many produce six or more output channels (including a sub-woofer output). This is in contrast to the two channel systems (i.e. left and right signals) which were common in the past. Providing five or more duplicate power supply circuits for each power amplifier within a single audio amplifier increases both the size and cost of the audio amplifier.
Furthermore, none of these devices provide for protection of the amplification from over-current, over-temperature or other overload conditions. Such protection is essential for practical commercial use of a power amplifier circuit.
Accordingly, there is a need for a power amplifier for audio signals that provides an efficient power supply with low EMI emissions and with low headroom between the power supplied to the amplification circuit and the output signal of the power amplifier. It is preferable if the power amplifier has a predictive control system that allows the headroom to be reduced while ensuring that sufficient power is provided to the amplification circuit (or circuits) at all times. It is also preferable that the control circuit and regulation system of the power amplifier be adaptable for use with multiple channels. It is also desirable that the power amplifier be adaptable to protect the amplification circuit of each channel so as to prevent the amplification circuit from being damaged by over-current, over-temperature or other overload conditions.
In a first embodiment, the present invention provides: a power amplifier for receiving an input signal at an input terminal and producing an output signal at an output terminal, the output signal corresponding to the input signal, the power amplifier having a first power supply circuit comprising: an amplifier coupled to the input terminal for receiving the input signal and coupled to the output terminal for providing the output signal, the amplifier having a power input terminal for receiving a power input signal; a switching regulator coupled to the power input terminal for providing a switching power signal to the amplifier, wherein the switching power signal forms a first part of the power input signal; a linear regulator coupled to the power input terminal, the linear regulator being selectively engageable to provide a linear power signal to the amplifier, wherein the linear power signal forms a second part of the power input signal; an input signal processing circuit coupled to the input terminal for receiving the input signal and for providing a rectified signal indicating the amount of power required by the amplifier; a control circuit coupled to the input signal processing circuit and to the power input terminal for controlling the switching power signal and the linear power signal in response to an error signal corresponding to the rectified signal and the power input signal; a linear regulator control circuit coupled to the input signal processing circuit for receiving the rectified signal and coupled to the linear regulator for controlling the engagement of the linear regulator in response to the rectified signal.
In a second embodiment, the present invention provides a power amplifier for receiving an input signal at an input terminal and producing an output signal at an output terminal, the output signal corresponding to the input terminal, the power amplifier having a first power supply circuit comprising: an EMI isolation circuit coupled to the input terminal for receiving the input signal and to an internal input node for providing an EMI-decoupled signal corresponding to the input signal an amplifier coupled to the input terminal for receiving the input signal and coupled to the output terminal for providing the output signal, the amplifier having a power input terminal for receiving a power input signal; a switching regulator coupled to the power input terminal for providing a switching power signal to the amplifier, wherein the switching power signal forms a first part of power input signal; a linear regulator coupled to the power input terminal, the linear regulator being selectively engageable to provide a linear power signal to the amplifier, wherein the linear power signal forms a second part of the power input signal; an input signal processing circuit coupled to the internal input node for receiving the EMI-decoupled signal and for providing a rectified signal indicating the amount of power required by the amplifier; a control circuit coupled to the internal input signal processing circuit and to the power input terminal for controlling the switching power signal and the linear power signal in response to an error signal corresponding to the rectified signal and the power input signal; a linear regulator control circuit coupled to the input signal processing circuit for receiving the rectified signal and coupled to the linear regulator for controlling the engagement of the linear regulator in response to the rectified signal.
A power amplifier for receiving a first input signal at a first input terminal and for producing a first output signal at a first output terminal, the first output signal corresponding to the first input signal, a first signal amplifier being coupled to the first input terminal to receive the first input signal and coupled to the first output terminal to provide the first output signal; the first signal amplifier having a first power terminal for receiving a total power signal and the power amplifier having a first power supply circuit comprising: a first input signal compensation block coupled to the first input terminal to receive the first input signal and to provide a compensated input signal corresponding to the first input signal, wherein the compensated input signal defines a target power level; a power signal compensation block for receiving the total power signal and for providing a compensated power signal corresponding to the total power signal; a summer coupled to the first input signal compensation block and to the power signal compensation block for providing an error signal corresponding to a difference between the target power level and a power level of the total power signal; a control circuit coupled to the summer for receiving the error signal and for providing a first control signal and a second control signal in response to the error signal, wherein the first control signal corresponds to a target main power signal level and the second control signal corresponds to a target transient power signal level; a transient detect block coupled to the first input signal compensation block for providing a transient signal to identify a transient condition when a rate of change in a slew rate of the compensated input signal exceeds a selected transient threshold; a main power supply for providing a main power signal at the first power terminal in response to the first control signal; and a selectively engageable transient power supply for providing a transient power signal at the first power terminal in response to the second control signal and the transient signal, wherein the transient power supply is engaged when the transient signal indicates that a transient condition exists; wherein the control circuit provides the first and second control signals such that the target main power signal level is equal to or higher than the target transient power signal level and wherein the magnitude of the total power signal is generally equal to the higher of the magnitude of the main power signal or the magnitude of the transient power signal.
A power amplifier for receiving a first input signal at a first input terminal and for producing a first output signal at a first output terminal, the first output signal corresponding to the first input signal, a first signal amplifier being coupled to the first input terminal to receive the first input signal and coupled to the first output terminal to provide the first output signal, the first signal amplifier having a first power terminal for receiving a total power signal and the power amplifier having a first power supply circuit comprising: a first input signal compensation block coupled to the first input terminal to receive the first input signal and to provide a compensated input signal corresponding to the first input signal, wherein the compensated input signal defines a target power level; a power signal compensation block for receiving the total power signal and for providing a compensated power signal corresponding to the total power signal; a summer coupled to the first input signal compensation block and to the power signal compensation block for providing an error signal corresponding to a difference between the target power level and the power level of the total power signal; a transient detect block coupled to the first input signal compensation block for providing a transient signal to identify a transient condition when a rate of change in a slew rate of the compensated input signal exceeds a selected transient threshold; a first transient control circuit coupled to the transient detect block for providing first and second digital transient control signals, wherein the first digital transient control signal indicates the occurrence of a transient condition for a first time period in response to the transient signal and wherein the second digital transient control signal indicates the occurrence of a transient condition for a second time period in response to the transient signal, and wherein the second time period is longer than the first time period; a control circuit coupled to the summer for receiving an amplified error signal for providing a first control signal in response to the amplified error signal; a signal combining block for combining the first control signal and the first transient control signal to provide a main power supply control signal; a selectively engageable second transient control circuit coupled to the first transient control circuit for receiving the second digital transient control signal and for temporarily increasing the magnitude of the error signal, wherein the second transient control circuit is engaged and disengaged in response to the second digital transient control signal, the second transient control circuit including a feedback amplifier coupled between the summer and the control circuit to provide the amplified error signal, the feedback amplifier being operative at all times; and a main power supply for providing a main power signal at the first power terminal in response to the main power supply control signal; wherein the total power signal corresponds to the main power signal.
A method of supplying a total power signal to a signal amplifier, comprising: receiving an input signal; producing a compensated input signal corresponding to the input signal, the compensated input signal defining a target power level for the total power signal; comparing the compensated input signal to a reduced version of the total power signal to produce an error signal; providing first and second control signals in response to the error signal; providing a main power signal using a switching regulator in response to the first control signal, the main power signal being a first part of the total power signal; comparing a rate of change of the compensated input signal to a selected transient threshold to provide a transient signal, the transient signal identifying a transient condition when the rate of change exceeds the transient threshold, the transient threshold corresponding to a maximum slew rate of the main power signal; and engaging a transient power supply to provide a transient power signal in response to the second control signal, when the transient signal indicates the transient condition, the transient power signal being a second part of the total power signal.
A method of supplying a total power signal to a signal amplifier, comprising: receiving an input signal; producing a compensated input signal corresponding to the input signal, the compensated input signal defining a target power level for the total power signal; comparing the compensated input signal to a reduced version of the total power signal to produce an error signal; providing first and second control signals in response to the error signal; providing a main power signal using a switching regulator in response to the first control signal, the main power signal being a first part of the total power signal; comparing a rate of change of the compensated input signal to a selected transient threshold to provide a transient signal, the transient signal identifying a transient condition when the rate of change exceeds the transient threshold, the transient threshold corresponding to a maximum slew rate of the main power signal; and in response to a transient condition, temporarily engaging the switching regulator with a 100% duty cycle for a first time period and temporarily, elevating the error signal for a second time period.
A power amplifier for receiving a first input signal at a first input terminal and for producing a first output signal at a first output terminal, the first output signal corresponding to the first input signal, a first signal amplifier being coupled to the first input terminal to receive the first input signal and coupled to the first output terminal to provide the first output signal, the first signal amplifier having a first power terminal for receiving a total power signal and said power amplifier having a first power supply circuit comprising: a first input signal compensation block coupled to the first input terminal to receive the first input signal and to provide a compensated input signal corresponding to the first input signal, wherein the compensated input signal defines a target power level; a main power signal compensation block for receiving a main power signal and for providing a compensated main power signal corresponding to the main power signal; a first summer coupled to the first input signal compensation block and to the main power signal compensation block for providing a first error signal corresponding to a difference between the target power level and a power level of the main power signal; a first control circuit coupled to the first summer for receiving the first error signal and for providing a first control signal in response to the first error signal, wherein the first control signal corresponds to a target main power signal level; a total power signal compensation block for receiving the total power signal and for providing a compensated total power signal corresponding to the total power signal; a second summer coupled to the first input signal compensation block and to the total power signal compensation block for providing the second error signal corresponding to a difference between the target power level and a power level of the total power signal; a second control circuit coupled to the second summer for receiving the second error signal and for providing a second control signal in response to the second error signal, wherein the second control signal corresponds to a target transient power signal level; a transient detect block coupled to the first input signal compensation block for providing a transient signal to identify a transient condition when a rate of change in a slew rate of the compensated input signal exceeds a selected transient threshold; a main power supply for providing a main power signal at the first power terminal in response to the first control signal; and a selectively engageable transient power supply for providing a transient power signal at the first power terminal in response to the second control signal and the transient signal, wherein the transient power supply is engaged when the transient signal indicates that a transient condition exists; wherein the magnitude of the total power signal is generally equal to the higher of the magnitude of the main power signal or the magnitude of the transient power signal.